The invention relates to an electrical circuit for the calibration of an apparatus (a measuring apparatus) which measures the volume of particles that flow through a measuring aperture in the stream of an electrolyte. Electrodes are disposed on both sides of the measuring aperture and the passage of particles through the aperture produces pulses which are evaluated in a data analysis unit. In the calibration process, a certain particle volume is associated arithmetically with the calibration pulses fed to the data analyzing unit.
Accordingly, the invention relates to an apparatus for the calibration of measuring apparatus for measuring particle volume according to the "Coulter" process, U.S. Pat. No. 2,656,508. Such apparatus is known in different embodiments, e.g., compare Kachel, Methods for the analysis and correction of apparatus-induced measuring errors in an electronic process for the determination of the size of particles according to Coulter, Berlin Dissertation 1972; Thom, "Comparative studies of electronic cell volume analysis", Published by A.E.G.-Telefunken, 1972; and DAS 1,806,512 and 2,013,799. Also known are circuits of the type described above for the calibration of such measuring apparatus (for instance Kachel, op.cit. page 55; Metzger, Valet, Kachel, Ruhenstroth-Bauer, "Blut," Volume 25, pages 179-184, 1972; Gutmann, "Elektromedizin" Volume 11, page 62, 196). Such apparatus or processes do not have the disadvantage of requiring calibration with particles of normalized size (Thom, Hampe, Sauerbrey, Z. ges. exp. Med., 151, pages 331-349, 1969) which is due to the fact that the accuracy of the data given by the manufacturer with respect to the dimensions of the normalized particles is not entirely reliable.
The known electrical calibration processes are based on the formula ##EQU1## according to which each artifically produced resistance change .DELTA.R is associated, by means of a measuring aperture simulated by an electrical resistance, with that volume which a particle woul have if it produced the same resistance change when passing through the measuring aperture. In the formula:
V IS THE VOLUME OF A PARTICLE PASSING THROUGH THE MEASURING APERTURE
V is the volume of the measuring aperture (length x cross-sectional area)
R is the electrical resistance of the measuring aperture when no particle is passing through it and
.DELTA.R is the resistance change of the measuring aperture when a particle is passing through it.
When a particular volume v is associated arithmetically with a particular resistance change .DELTA.R, consideration must also be given to a form factor (form of the particle) and a so-called capillary factor relating to the shape of the measuring aperture; however, the influence of these factors is applicable to the known apparatus in the same measure as for the apparatus according to the invention. For this reason, they need not be separately considered in the present connection.
The known electrical circuts (Kachel, Op. Cit. page 55) start by producing a resitance change .DELTA.R at the input of the electronic portion of the measuring apparatus, i.e., the data analyzing apparatus. The value of this resistance change must be precisely defined with respect to the resistance of the actual measuring aperture and, for this reason, the resistance of the measuring aperture itself is simulated by another resistor. Thus, a resistance is applied to the input of the electronic part of the data analyzing unit which represents the resistance of the measuring aperture and this resistance is changed in a well-defined manner. Based on this resitance change and with the use of Formula (1), a particular particle volume is calculated and is associated with the voltage pulses received by the data analyzing unit and caused by the resistance change .DELTA.R.
A process of this type has several disadvantages: first of all, a simulation of the resistance of the measuring aperture is cumbersome because the true resistance of the measuring aperture must first be determined and simulated. During the calibration, the measuring aperture must be uncoupled from the data analyzing unit and the simulator must be attached. Now, from the point of circuit design, it is extremely difficult to produce resistance changes of approximately 0.1 to 0.01 percent in resistances of the order of magnitude of 10 kilo-ohms at repetition frequencies in the region of several kilohertz. When relays are used for switching in resistance changes, the resulting pulse shape is fixed as rectangular but this does not correspond to the true bell-shaped or trapezoidal form of the measuring pulses, which can lead to falsifications. When voltage-sensitive amplifiers are used in the analysis unit, it is necessary to simulate the resistance of the measuring aperture exactly and this is due to the fact that this resistance, together with the shielding capacitances, the further capacitances in the measuring system, as well as the capacitances of the input of the amplifier together form an RC member in the analysis unit which lengthens the rise time of the amplifier. In the case of very short pulses, this effect can even lead to a reduction of the amplitude and hence to a falsification of the calibration process, since the voltage pulses registered in the analysis unit must be identical during measuremment and calibraion. Furthermore, aside from this effect, deviations of the simulated resistance of the measuring aperture from its true resistance would be tolerable up to a limit of a few percent when using voltage-sensitive amplifiers, whereas, when using current-sensitive amplifiers, it is necessary to simulate the resistance of the measuring aperture with especially high precision because the magnitude of the input resistance directly influences the gain of such amplifiers.